It is well known to blend a rubber with a hard resin to obtain an impact resistant resin-modified with the rubber. An ABS resin is a typical resin of this type. However, since the heat resistance of the acrylonitrile-styrene copolymer (AS resin) as the hard resin is inadequate, it has been desired to have a hard resin having a higher heat deflection temperature.
As a styrene resin having high heat deflection temperature, for example, a copolymer of an N-substituted maleimide monomer with an aromatic vinyl monomer is known (L. E. Coleman et al., J. Polymer Sci., Vol. 38, p. 241, 1959). Further, a graft copolymer obtained by grafting a monomer mixture comprising an N-allyl-substituted maleimide, an aromatic vinyl monomer and acrylonitrile to a conjugated diene rubber and a resin composition containing such a graft copolymer have been proposed (U.S. Pat. No. 3,652,726).
However, the copolymer of an N-substituted maleimide monomer with a vinyl monomer tends to be brittle as the N-substituted maleimide monomer content increases, although the heat deflection temperature may thereby be made high. In order to overcome the brittleness, it has been proposed to blend an ABS resin (Japanese Unexamined Patent Publications Nos. 98536/1982 and 131213/1982), but such a proposal is still not satisfactory.
In order to improve the impact resistance of such a composition comprising an N-substituted maleimide styrene copolymer and an ABS resin, it is necessary to increase the amount of the ABS resin in the composition or to increase the proportion of the graft rubber in the ABS resin to be blended. On the other hand, the mechanical properties such as heat resistance and rigidity deteriorate with an increase of the amount of the ABS resin or with an increase of the proportion of the graft rubber in the ABS resin. Therefore, it is desired to develop a resin composition of an N-substituted maleimide-styrene copolymer which is excellent in both the heat resistance and the impact resistance.